Lexus Forum - Lexus Enthusiast Forums - Check Engine Light Turned On ?

I got a Lexus IS300 and the light wasn't on. After I changed my original exhaust to another exhaust (its factory from another Lexus IS300, not after market), then sometime after that, the light is on so I don't really think its the O2 sensor. I took it to autozone and the guy plugged the computer into my car and found 3 possibilities, one of them is catalytic converter. So I was think that is the catalytic converter malfunction or that the guy didn't properly install the exhaust. Does anyone know anything or have similar experience? Please help or recommend. Thanks